The efficiency of μ-opioid receptor signalling is tightly regulated and ultimately limited by the coordinated
phosphorylation of intracellular serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific μ-opioid receptor antibodies, which have proved to be excellent tools for monitoring the spatial and temporal dynamics of receptor
phosphorylation and de
phosphorylation. Agonist-induced
phosphorylation of μ-opioid receptors occurs at a conserved 10 residue sequence
370TREHPSTANT
379 in the receptor''s carboxyl-terminal cytoplasmic tail. Diverse opioids induce receptor
phosphorylation at S375, present in the middle of this sequence, but only high-efficacy opioids have the ability to drive higher order
phosphorylation on flanking residues (T370, T376 and T379). S375 is the initiating residue in a hierarchical
phosphorylation cascade. In contrast, agonist-independent heterologous μ-opioid receptor
phosphorylation occurs primarily at T370. The combination of phosphosite-specific antibodies and siRNA knockdown screening also facilitated the identification of relevant kinases and phosphatases. In fact, morphine induces a selective S375
phosphorylation that is predominantly catalysed by GPCR kinase 5 (GRK5), whereas multisite
phosphorylation induced by high-efficacy opioids specifically requires GRK2/3. By contrast, T370
phosphorylation stimulated by phorbol esters or heterologous activation of G
q-coupled receptors is mediated by PKCα. Rapid μ-opioid receptor de
phosphorylation occurs at or near the plasma membrane and is catalysed by protein phosphatase 1γ (PP1γ). These findings suggest that there are distinct
phosphorylation motifs for homologous and heterologous regulation of μ-opioid receptor
phosphorylation. However, it remains to be seen to what extent different μ-opioid receptor
phosphorylation patterns contribute to the development of tolerance and dependence
in vivo.
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This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit
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